Chapter  11:  RNA Processing, Nuclear Transport, and Post-Transcriptional Control

The non-snRNP pre-mRNA splicing factor SC35 localizes in a speckled distribution in interphase nuclei (orange regions). HeLa cell SC35 was visualized by immuno-staining with a fluorescently labeled antibody. An optical section of the immunostaining pattern is superimposed over a differential interference contrast image of the cells. [Photograph courtesy of David L. Spector, Cold Spring Harbor Laboratory.]

In the previous chapter, we saw that regulation of most genes occurs at the first step in gene expression, namely, initiation of transcription. However, once transcription has been initiated, synthesis of the encoded RNA requires that RNA polymerase transcribe the entire gene and not terminate prematurely. The initial primary transcripts produced from eukaryotic genes are not functional and undergo various processing reactions to yield the corresponding functional RNAs. The mature, functional RNAs in the nucleus, then are actively transported to the cytoplasm, as components of ribonucleoproteins.

The multiple steps in the production of RNAs provide opportunities for additional levels of gene control beyond the regulation of transcription initiation. In the case of a protein-coding gene, the amount of protein expressed also can be regulated by controlling the stability of the corresponding mRNA in the cytoplasm and the rate of its translation. In addition, the cellular locations of some mRNAs are regulated, so that newly synthesized protein is concentrated where it is needed. All of the regulatory mechanisms that control gene expression following transcription initiation are referred to as post-transcriptional control. In this chapter, we consider the various steps in the synthesis of mRNA, tRNA, and rRNA following transcription initiation and the mechanisms by which RNAs and proteins are transported in and out of the nucleus in eukaryotic cells. We also present relevant examples of how regulation of these steps contributes to the control of gene expression.